The present invention relates generally to automotive dampers or shock absorbers which receive mechanical shock. More particularly, the present invention relates to a unique hydraulic valve assembly which allows greater tunability of the shock absorber, especially in the mode of low hydraulic fluid flow.
Shock absorbers are used in conjunction with automotive suspension systems to absorb unwanted vibrations which occur during driving. To absorb unwanted vibrations, shock absorbers are generally connected between the sprung portion(body) and the unsprung portion (suspension) of the automobile. A piston is located within a pressure tube of the shock absorber and is connected to the sprung portion of the automobile through a piston rod. The piston divides the pressure tube into an upper working chamber and a lower working chamber. Because the piston is able, through valving, to limit the flow of damping fluid between the upper and lower working chambers when the shock absorber is compressed or extended, the shock absorber is able to produce a damping force which counteracts the vibration which would otherwise be transmitted from the unsprung portion to the sprung portion of the automobile. In a dual tube shock absorber, a fluid reservoir is defined between the pressure tube and the reservoir tube. A base valve is located between the lower working chamber and the reservoir to limit the flow of fluid between the lower working chamber and the reservoir to produce a damping force which also counteracts the vibration which would otherwise be transmitted from the unsprung portion to the sprung portion of the automobile. The greater the degree to which the flow of fluid within the shock absorber is restricted by the piston valving or the base valve, the greater the damping forces which are generated by the shock absorber. Thus, a highly restricted flow of fluid would produce a firm ride while a less restricted flow of fluid would produce a soft ride.
In selecting the amount of damping that a shock absorber is to provide, at least three vehicle performance characteristics are considered. These three characteristics are ride comfort, vehicle handling and road holding ability. Ride comfort is often a function of the spring constant of the main springs of the vehicle as well as the spring constant of the seat, tires and the damping coefficient of the shock absorber. For optimum ride comfort, a relatively low damping force or a soft ride is preferred. Vehicle handling is related to the variation in the vehicle""s attitude (i.e. roll, pitch and yaw). For optimum vehicle handling, relatively large damping forces or a firm ride are required to avoid excessively rapid variations in the vehicle""s attitude during cornering, acceleration and deceleration. Road holding ability is generally a function of the amount of contact between the tires and the ground. To optimize road handling ability, large damping forces are required when driving on irregular surfaces to prevent loss of contact between the wheel and the ground for excessive periods of time.
Various types of shock absorbers have been developed to generate the desired damping forces in relation to the various vehicle performance characteristics. Shock absorbers have been developed to provide different damping characteristics depending on the speed at which the piston moves within the pressure tube. Because of the exponential relation between pressure drop and flow rate for a fixed orifice, it is difficult to obtain damping force at low piston rod velocities, particularly at velocities near zero. Low speed damping force is important to vehicle handling since most vehicle handling events are controlled by low speed body velocities.
One known method for tuning shock absorbers during low speed movement of the piston is to create the low speed bleed orifice by utilizing open orifice notches positioned either on the flexible disc adjacent to the sealing land or in the sealing land itself. The configuration of these open orifices is therefore constant and is not a function of the internal pressures. To obtain the low speed control utilizing these open orifice notches, the orifice notches have to be small enough to create the orifice restriction at low velocities. When this is accomplished, the low speed circuit of the valving system operates over a very small range of velocity, therefore the second stage valving is activated at a relatively low velocity. Activation of the second valving stage at relatively low velocities creates harshness because of the shape of the fixed orifice bleed circuit force velocity characteristic which is much different in configuration than the shape of the mid-speed circuit.
Prior art attempts at limiting harshness during low speed piston movements have included the incorporation of a variable orifice bleed valving circuit. As the velocity of the piston increases, the flow area of the variable orifice bleed disc increases. The prior art variable orifice bleed valving orifice area is opened by the outside diameter of the valving disc. Thus, the diameter of the disc determines the rate at which the flow area increases. As the diameter of the disc increases, it becomes difficult to control the rate at which the flow area of the orifice increases. The flow area is increased by the deflection of the variable orifice bleed disc. Small deflection of the large diameter variable orifice bleed discs provides a rapid increase in the flow area of the bleed orifice. The rapid increase in the flow area complicates the tuning between the low speed valving circuit and the secondary circuit.
Continued development of shock absorbers include the development of a valving system which can smooth the transition between the low speed valving system and the second stage valving system in order to reduce and/or eliminate the harshness during the transition.
The present invention provides the art with a method for tuning damping forces at low piston velocities in an effort to improve the handling characteristics of the vehicle without creating harshness. The present invention provides a ported disc variable orifice bleed circuit which incorporates a pressure sensitive orifice that is located down stream from three fixed orifices radially located in the ported restriction disc. Once hydraulic fluid flows through the fixed orifice, it flows into three pressure areas in the same disc. These pressure areas provide the hydraulic fluid with a means of acting on the variable orifice bleed circuit. The size of the pressure area and the preload on the low speed disc dictate the pressure needed to open the low speed disc. This feature provides the necessary tunability for the present invention.
Other advantages and objects of the present invention will become apparent to those skilled in the art from the subsequent detailed description, appended claims and drawings.